The significance of the firmness of a roll of web material, especially paper, with respect to the quality of the wound-up web has long been recognized. Consequently, various methods have been proposed to determine the roll firmness.
A known method for evaluating the firmness of a finished roll uses test strips which are inserted within the roll at different radii thereof between selected turns with their ends projecting towards the exterior. The force required to pull out the strips is a measure of the roll firmness.
Although providing a determination of the roll firmness, this method does not allow control thereof during formation of the roll in order to allow possible corrective measures if the roll firmness deviates from a desired value.
It has therefore been proposed to determine the roll firmness during the roll formation, i.e. the winding operation, in dependence on a change of the density. This method, however, yields only average values of the roll firmness over the width of the roll. Local alterations of the roll firmness, e.g. because of fluctuations of the web profile or uneven or unequal roller loads cannot, however, be detected.